Electric high-voltage fuse having means for effecting sequential vaporization of portions of fusible element



F. J. KOZACKA ELECTRIC HIGH-VOLTAGE FUSE HAVING MEANS FOR Jan. 3, 1967 EFFECTING SEQUENTIAL VAPORIZATION OF PORTIONS OF FUSIBLE ELEMENT Filed Oct. 19, 1965 I. F 0 0 O O O o 0 lCl 4 l E F I. I l G .w E T I a m l I I l l l A ll In L FIG. IA

FIG. 2A

INVENTOR FREDERICK J. KOZACKA United States Patent ELECTRIC HIGH-VOLTAGE FUSE HAVING MEANS FOR EFFECTING SEQUENTIAL VAPORIZATION OF PORTIONS OF FUSIBLE ELEMENT Frederick J. Kozacka, South Hampton, N.H., assignor to The Chase-Shawmut Company, Newburyport, Mass. Filed Oct. 19, 1965, Ser. No. 497,727 5 Claims. (Cl. 200120) This invention relates to high-voltage fuses. The term high-voltage fuses, as applied in this context, is limited to fuses designed for circuits having a circuit voltage in excess of 1000 volts, and having fusible elements whose length exceeds 3 inches.

Fusible elements for such fuses are generally provided with a plurality of serially related points of equally re- .duced cross-section. The larger the number of points of reduced cross-section per unit of length of the fusible element, the higher the arc voltage generated incident to blowing of the fuse, the larger the current-limiting action of the fuse, and the smaller its let-through current. In order to achieve these desirable ends it is, therefore, necessary to increase as much as possible the number of points of reduced cross-section per unit of length of the fusible element. Such an increase of the number of points of reduced cross-section of the fusible element tends to result in dangerous voltage spikes at the time of arc initiation followed by an extremely rapid decay of the arc voltage. These are dangerous and even intolerable performance characteristics, compelling to keep the number of points of reduced cross-section within certain limits. Such limits are a compromise between conflicting requirements and are not compatible with the highest performance standards.

The fusible elements having points of relatively small cross-sectional area alternating with points of relatively large cross-sectional area were introduced into the highvoltage fuse art in order to achieve sequential vaporization of the first mentioned points and of the last mentioned points, and to thereby limit the rate of change of the fault current, and to thereby limit the inductive surge voltage which is proportional to the rate of change of the fault current. The peak of the inductive surge voltage is proportional to the number of points of reduced cross-section of the fusible element per unit of length multiplied by the total length thereof. Therefore any increase of the number of points of reduced cross-section of a fusible element beyond a certain range defeats the very purpose of the use in high-voltage fuses of fusible elements having points of relatively small cross-section alternating with points of relatively large cross-section.

High-voltage fuses are generally relatively long and have generally fuse tubes of relatively small diameter, Whereas low-voltage fuses are generally relatively short and haveas far as power applications are concerned-fuse tubes of relatively large diameter. For these reasons the ratio of radial heat flow to axial heat flow tends to be relatively large in high-voltage fuses and relatively small in low-voltage fuses. Therefore high-voltage fuses carrying current tend to have a flattop temperature distribution along their fusible element or elements, and low-voltage fuses tend to have a substantially parabolic temperature distribution along their fusible element or elements.

A virtually absolute flat-top temperature distribution is achieved in high-voltage fuses whose fusible element is straight and of uniform cross-section, i.e. in such fuses the temperature along the center region of the fusible element is substantially equal and decreases only in axial direction toward the ends of the fusible element, or at the terminal elements of the fuse structure. If the 3,296,399 Patented Jan. 3, 1967 fusible element has points of relatively small cross-section alternating with points of relatively large cross-section, the flat-top character of the temperature distribution curve still prevails, but the actual temperature distribution curve comprises a series of peaks extending above the fiat-top level. The height of these peaks varies with the current, but is always substantially equal. As a result, all points of equal cross-section fuse and vaporize substantially simultaneously, resulting in an inductive surge voltage peak which is proportional to the number of points which fuse and vaporize simultaneously.

The virtually parabolic temperature distribution is only achieved in low-voltage fuses whose fusible element is straight and of uniform cross-section, i.e. in such fuses which carry current the temperature has a peak value in the center of the fusible element and decreases progressively toward the ends of the fusible element, or toward the terminal elements of the fuse structure. If the fusible element has points of relatively small cross-section alternating with points of relatively large cross-section, the parabolic character of the temperature distribution curve still prevails, but the actual temperature distribution curve comprises a series of peaks extending above, or superimposed upon, the basically parabolic temperature distribution curve. The height of these peaks varies with the load current. If a major fault current occurs substantially the same amount of heat is generated at all points of the fusible element whose cross-section is equal. Under such circumstances the amount of heat dissipated from all points of the fusible element whose cross-section is equal is negligible. Since the temperature of the :points of equal cross-section of the fusible element differs at the time of fault inception, and since fault inception adds substantially the same amounts of heat to all points of the fusible element whose crosssection is equal, points of equal cross-section of the fusible element reach their fusing temperature and their vaporization temperature at different times, i.e. these points tend to fuse and vaporize sequentially under major fault current conditions.

In high-voltage fuses having a substantially flat-top temperature distribution along the fusible element, or fuse link, thereof additional means must often be used to preclude all, or most of the points of reduced crosssection, from fusing and vaporizing substantially simultaneously, thereby generating an excessive arc voltage in the circuit under interruption. The common means applied to avoid generation of an excessive arc voltage is the so-called graded fusible element, or fuse link. A graded fusible element, or fuse link, includes a plurality of serially related points of minimum cross-section, a plurality of serially related points of intermediate crosssection and a plurality of serially related points of maximum cross-section. The various serially related points of such a fusible element, or fuse link, fuse and vaporize in the sequence of their crosssection, beginning with the smallest cross-section. This establishes a means of controlling the arc voltage in the circuit under interruption.

Manufacturing graded fusible elements, :or fuse links, requires special relatively expensive dies. For this reason it is not economically feasible to manufacture fuses having graded fusible elements, or fuse links, unless a relatively large quantity of fuses is required. It is, however, often necessary and desirable to manufacture fuses for special applications in relatively small quantities and without graded fusible elements, and to impart to such fuses the kind of control of the generation of arc voltage characteristic of fuses having graded fusible elements.

It is a general object of this invention to provide improved current-limi-ting high-voltage fuses.

Another object of this invention is to provide currentlimiting high-voltage fuses which make it possible to control the generation and the peak value of the arc voltage.

Another object of this invention is to provide currentlimiting high-voltage fuses which do not include grade-d fuse links but perform in the way of fuses which include graded fuse links.

Still another object of this invention is to provide current-limiting fuses which perform satisfactorily both in the range of major fault currents and in the range of protracted overload currents.

Other objects of the invention and advantages thereof will, in part, be obvious and in part appear hereinafter.

For a more complete understanding of the invention reference may be had to the following description thereof taken in connection with the accompanying drawings in which:

FIG. 1 is in part a front View and in part a longitudinal section of a fuse embodying this invention intended to carry relatively small currents and to afford protection against major fault currents only, rather than against both overload currents and major fault currents, or short-circuit currents;

FIG. 1A is a section along 1A1A of FIG. 1;

FIG. 2 is in part a front view and in part a longitudinal section of a fuse embodying this invention intended to carry relatively large currents and to afford protection against protracted overload currents and against major fault currents, or short-circuit currents; and

FIG. 2A is a section along 2A-2A of FIG. 2.

Referring now to the drawings, and more particularly to FIGS. 1 and 1A thereof, numeral 1 has been applied to indicate a fuse tube of insulating material, e.g. of a melamine-glass-cloth laminate. Fuse tube 1 is filled with a rnass 2 of quartz sand. Other arc-quenching fillers which are acceptable in low-voltage fuses are not acceptable for fuses embodying this invention. Fuse tube 1 is closed on both ends thereof by terminal elements 3 in the form of caps or ferrules. Caps or ferrules 3 are firmly held in position by drive screws 4 projecting into fuse tube 1. Fusible element 5 of considerable length is arranged inside of fuse tube 1. The fusible element 5 shown in FIG. 1 is 9 inches long and intended for cirouits having a circuit voltage of 2000 volts and to carry continually currents of 30 amps. It is formed by a ribbon of silver of uniform width having a plurality of serially related circular perforations forming serially related points of reduced and equal cross-section. The axially outer ends of fusible element or fuse link 5 are conductively connected to terminal elements 3 and thus fusible element 5 conductively interconnects terminal elements 3. Each terminal element 3 is providedwith a narrow slit for the passage of one end of fusible element 5, and each terminal element 3 defines a recess 3a into which one of the axially outer ends of fusible element 5 projects and which. holds a mass 312 of solder establishing an electric connection between fusible element 5 and the respective terminal element 3. Reference numeral 6 has been applied to indicate a sub-casing of thermal insulating material, e.g. a cellulosic substance, which is a particularly good thermal insulator, arranged inside of fuse tube 1 in coaxial relation thereto and spaced from fuse tube 1. The sub-casing 6 comprises a tubular portion which is closed on both ends thereof by a pair of caps. These caps have slits allowing the passage of ribbon fuse link 5. All outer surfaces of sub-casing 6 are submersed in the mass 2 of quartz sand. Subcasing 6 encloses a length, or first portion, of fusible element 5 including a portion of the total number of points of reduced cross-section of fuse link 5. The portion of link 5 inside sub-casing 6 is spaced from the lateral wall of sub-casing 6. Since the air space surrounding said first portion of points of reduced crosssection of fuse link 5 is a relatively good thermal insulator, the first or air immersed portion of points of reduced cross-section operates at a relatively high temperature level when fusible element 5 is carrying current. Fusible element or fuse link 5 further includes a second portion of its total number of points of reduced crosssection which is situated outside of sub-casing 6 and directly immersed in the mass 2 of quartz sand. Since the mass 2 of quartz sand in physical engagement with this second portion of points of reduced cross-section of link 5 is a relatively good conductor of heat, this second portion of points of reduced cross-section of fusible element 5 operates at a relatively low temperature when the fusible element 5 is current-carrying.

On occurrence of major fault currents the points of reduced crosssection of fuse link 5 which are enclosed in sub-casing 6 will fuse and vaporize before any other points of the fuse link fuse and vaporize. At a subsequent point of time the points of reduced cross-section of fuse link 5 which are outside of sub-casing 6 will fuse and vaporize before the points of the fuse link outside of sub-casing 6 whose cross-section is not reduced fuse and vaporize. Following fusion and vaporization of the points of reduced cross-section outside of sub-casing 6, the points of the fuse link 5 outside of sub-casing 6 whose cross-section is not reduced will fuse and vaporize.

This sequence of steps in the interrupting process makes it possible to carefully control the generation of arc voltage and to avoid dangerous voltage surges without resorting to the conventional means of a graded fuse link.

The fuse structure of FIGS. 1 and 1A must be protected by another protective device, e.g. a circuit breaker which is arranged in series with it, against protracted overload currents. If such additional protection were lacking, the fusible element might reach a temperature at the occurrence of protracted overload currents sufficiently high to damage or destroy sub-casing 6 and/or fuse tube 1.

The structure of FIGS. 1 and 1A may, however, be readily adapted to cope with protracted overload currents, as will be more fully explained in connection with FIGS. 2 and 2A.

In FIGS. 2 and 2A the same reference characters as in FIGS. 1 and 1A, however, with a prime added, have been applied to indicate like parts. Thus reference numeral 1 has been applied to indicate a fuse tube of insulating material filled with a mass of quartz sand 2'. Fuse tube 1' is closed on the ends thereof by terminal elements 3', held in position by drive screws 4'. Terminal elements 3 are formed by caps or ferrules which are conductively interconnected by a plurality of straight ribbon fuse links 5. A plurality of points of reduced crosssection of each multiperforated fuse link 5' is arranged inside of sub-casing 6' made of an organic insulating material. Sub-casing 6 is made-up of a tubular element and a pair of caps having slits for the passage of fuse links 5'. It is arranged in coaxial relation to fuse tube 1' and entirely surrounded by the mass 2 of quartz sand. Sub-casing 6' defines a void which has a very small thermal conductivity compared to that of quartz sand. It is sufficiently tight to preclude the mass 2 of quartz sand from getting access to the points of reduced crosssection of fuse links 5' situated inside of sub-casing 6', yet is not so tight as to preclude products of arcing formed upon vaporization of the points of reduced crosssection of links 5 inside sub-casing 6' from flowing out of it and into the mass 2' of quartz sand by which it is surrounded.

Due to the length of fuse links 5, the points of reduced cross-section thereof inside sub-casing 6 operate at a relatively high substantially equal temperature level when fusible elements or fuse links 5 carry current. The points of reduced cross-section of fuse links 5 outside sub-casing 6' operate at a relatively low substantially equal temperature level when fusible elements 5 are carrying current.

Fusible elements or fuse links 5 are preferably of silver. One of the plurality of perforations in each portion of fusible elements 5' arranged inside of sub-casing 6' is used to maintain in position a rivet 7 of tin, or of a similar metal having a lower fusing point than the metal of which fuse links 5 are made and capable, upon fusion thereof, of severing fuse links 5 by a metallurgical reaction. Any desired overlay on links 5' of a lowfusing-point link-severing metal may be substituted for tin rivets 7', eg an overlay of indium, as disclosed and claimed in US. Patent 2,703,352 to Frederick J. Kozacka, issued March 1, 195-5, Fuse and Fuse Link of the Time Lag Type.

The rivets 7' on fuse links 5', or an equivalent linksevering overlay of a low-fusing-point metal thereon, preclude the temperature inside of sub-casing 6 from rising above a predetermined limit without blowing of the fuse, i.e. the temperature inside of sub-casing 6' rises only in case of blowing of the fuse above the aforementioned predetermined temperature. This affords an effective thermal protection for sub-casing 6' and fuse tube 1. Hence the fuse structure of FIGS. 2 and 2A is self-protecting, i.e. it does not require back-up protection against small protracted overload currents by another serially connected interrupting device.

Assuming the fuse shown in FIGS. 2 and 2A were not provided with sub-casing 6 and had, if so modified, a given current rating, or a given minimum fusing current. Provision of sub-casing 6 then results in a drastic derating of the fuse which is due to the decrease in thermal conductivity of the space surrounding the center regions of fuse links 5, and more particularly to the mutual heating of these center regions. In order to restore the same current rating, or minimum fusing current, the fuse had prior to the provision of sub-housing 6' and the void defined by the latter, the ribbon material of which fuse links 5 are made must he made considerably thicker. As a result, particularly long time delays are achieved, i.e. the time lag which elapses prior to blowing of the fuse in response to small protracted overload currents is greatly increased. In other words, in the structure of FIGS. 2 and 2A the sub-casing 6' performs two functions, i.e. it controls the surge-voltage generated incident to blowing of the fuse on major fault currents, or short-circuit currents, and it also greatly increases the time-lag when the fuse is caused to blow by protracted relatively small overload currents.

It will be understood that I have illustrated and described herein preferred embodiments of my invention and that various alterations may be made in the details thereof without departing from the spirit and scope of my invention as defined in the appended claims.

I claim:

1. An electric current-limiting high-voltage fuse for circuit voltages in excess of 1000 volts comprising in combination:

(a) an elongated fuse tube of insulating material;

(b) a mass of quartz sand inside said fuse tube;

(c) a pair of terminal elements closing the ends of said fuse tube;

(d) a relatively long fusible element inside said fuse tube conductively interconnecting said pair of terminal elements, said fusible element having a plurality of serially related points of reduced and equal crosssection; and

(e) a sub-casing of a thermal insulating material inside of and spaced from said fuse tube, all outer surfaces of said sub-casing being submersed in said mass of quartz sand, said sub-casing enclosing a first portion of said plurality of points of reduced crosssection of said fusible element immersed in air and segregated from said mass of quartz sand operating at a relatively high temperature level when said fusible element is carrying current, and a second portion of said plurality of points of reduced crosssection of said fusible element being situated outside said sub-casing and directly immersed in said mass of quartz sand and operating at a relatively low temperature level when said fusible element is carrying current.

2. An electric current-limiting high-voltage fuse for circuit voltages in excess of 1000 volts comprising in combination:

(a) an elongated fuse tube of insulating material;

(b) a mass of quartz sand inside said fuse tube;

(0) a pair of terminal elements closing the ends of said fuse tube;

(d) a relatively long straight fusible element inside said fuse tube conductively interconnecting said pair of terminal elements, said fusible element having a plurality of serially related points of reduced and equal cross-section:

(e) a sub-casing of a thermal insulating material being a poor heat conductor of substantially the same order as cellulosic materials inside of and spaced from said fuse tube, all outer surfaces of said sub-casing being submersed in said mass of quartz sand, said sub-casing enclosing a first portion of said plurality of points of reduced cross-section of said fusible element immersed in air and segregated from said mass of quartz sand operating at a relatively high substantially equal temperature level when said fusible element is carrying current, a second portion of said plurality of points of reduced cross-section of said fusible element being situated outside said sub-casing and directly immersed in said mass of quartz sand operating at a relatively low substantially equal temperature level when said fusible element is carrying current, and said sub-casing being sufficiently tight to preclude said mass of quartz sand from getting access to said first portion of said plurality of points of reduced cross-section, yet allowing products of arcing formed upon vaporization of said first portion of said plurality of points of reduced cross-section to flow into said mass of quartz sand.

3. An electric current-limiting high-voltage fuse for circuit voltages in excess of 1000 volts comprising in combination:

(a) an elongated fuse tube of insulating material;

(b) a mass of quartz sand inside said fuse tube;

(0) a pair of terminal elements closing the ends of said fuse tube;

(-d) a relatively long fuse link of a relatively high fusing point metal inside said fuse tube conductively interconnecting said pair of terminal elements, said fuse link having a plurality of serially related points of reduced and equal cross-section;

(e) a sub-casing of an organic thermal insulating material being a poor heat conductor of substantially the same order as cellulosic materials inside of and spaced from said fuse tu-be, all outer surfaces of said sub-casing being submerged in said mass of quartz sand, said sub-casing enclosing a first portion of said plurality of points of reduced cross-section of said fuse link immersed in air and segregated from said mass of quartz sand operating at a relatively high substantially equal temperature level when said fuse link is carrying current, a second portion of said plurality of points of reduced crosssection of said fuse link being situated outside said sub-casing and directly immersed in said mass of quartz sand and operating at a relatively low substantially equal temperature level when said fusible element is carrying current; and

(f) a mass of a relatively low fusing point link-severing metal on said fuse link adjacent one point of reduced cross-section thereof pertaining to said first portion of said plurality of points of reduced cross section.

4. An electric current-limiting high-voltage fuse for circ-uit voltages in excess of 1000 volts comprising in combination:

(a) an elongated fuse tube of insulating material;

(b) a mass of quartz sand inside said fuse tube;

(c) a pair of terminal elements closing the ends of said fuse tube;

(d) a plurality of relatively long, straight parallel fuse links of a relatively 'high fusing point metal inside said fuse tube conductively interconnecting said pair of terminal elements, each of said plurality of fuse links having a plurality of serially related points of reduced and equal cross-section;

(e) a sub-casing of organic thermal insulating material being a poor heat conductor of substantially the same order as cellulosic materials inside of and spaced from said fuse tube, all outer surfaces of said subcasing being submersed in said mass of quartz sand, said sub-casing enclosing a first portion of said plurality of points of reduced cross-section of each of said plurality of fuse links immersed in air and segregated from said mass of quartz sand, a second portion of said plurality of points of reduced cross-section of each of said plurality of fuse links being situated outside said sub-casing and directly immersed in said mass of quartz sand; and

(f) a plurality of masses of a relatively low fusing point link-severing metal each on one of said plurality of fuse links adjacent one point of reduced cross-section thereof pertaining to said first portion of said plurality of points of reduced cross-section.

5. An electric current-limiting high-voltage fuse for circuit voltages in excess of 1000 volts comprising in combination:

(a) an elongated fuse tube of insulating material;

(b) a mass of quartz sand inside said fuse tube;

(c) a pair of terminal elements closing the ends of said fuse tube;

(d) a plurality of relatively long, straight parallel fuse links of a relatively high fusmg point metal inside said fuse tube conductively interconnecting said pair of terminal elements, each of said plurality of fuse links having a plurality of serially related points of reduced and equal cross-section;

(e) a sub-casing of organic thermal insulating material being a poor heat conductor of substantially the same order as cellulosic materials inside of and spaced from said fuse tube, all the outer surfaces of said sub-casing being submersed in said mass of quartz sand, each of said plurality of fuse links projecting in a direction longitudinally thereof through said subcasing and said sub-casing forming a common enclosure for only the center portion of each of said plurality of fuse links for immersing said center portion of each of said plurality of fuse links in air and for segregating said center portion of each of said plurality of fuse links from said mass of quartz sand, said sub-casing allowing products of arcing formed upon vaporization of said center portion of each of said plurality of fuse links to flow into said mass of quartz sand; and

(f) a plurality of masses of a relatively low fusing point link-severing metal each on said center portion of one of said plurality of fuse links.

References Cited by the Examiner UNITED STATES PATENTS 569,373 10/1896 DoWnes 200 2,665,348 1/ 1954 Kozacka 200- 131 X 2,808,487 10/1957 Jacobs 200l20 X 2,827,532 3/1958 Kozacka 200-181 2,832,868 4/ 8 Kozacka 200131 BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner. 

1. AN ELECTRIC CURRENT-LIMITING HIGH-VOLTAGE FUSE FOR CIRCUIT VOLTAGES IN EXCESS OF 1000 VOLTS COMPRISING IN COMBINATION: (A) AN ELONGATED FUSE TUBE OF INSULATING MATERIAL; (B) A MASS OF QUARTZ SAND INSIDE SAID FUSE TUBE; (C) A PAIR OF TERMINAL ELEMENTS CLOSING THE ENDS OF SAID FUSE TUBE; (D) A RELATIVELY LONG FUSIBLE ELEMENT INSIDE SAID FUSE TUBE CONDUCTIVELY INTERCONNECTING SAID PAIR OF TERMINAL ELEMENTS, SAID FUSIBLE ELEMENT HAVING A PLURALITY OF SERIALLY RELATED POINTS OF REDUCED AND EQUAL CROSSSECTION; AND (E) A SUB-CASING OF A THERMAL INSULATING MATERIAL INSIDE OF AND SPACED FROM SAID FUSE TUBE, ALL OUTER SURFACES OF SAID SUB-CASING BEING SUBMERSED IN SAID MASS OF QUARTZ SAND, SAID SUB-CASING ENCLOSING A FIRST PORTION OF SAID PLURALITY OF POINTS OF REDUCED CROSSSECTION OF SAID FUSIBLE ELEMENT IMMERSED IN AIR AND SEGREGATED FROM SAID MASS OF QUARTZ SAND OPERATING AT A RELATIVELY HIGH TEMPERATURE LEVEL WHEN SAID FUSIBLE ELEMENT IS CARRYING CURRENT, AND A SECOND PORTION OF SAID PLURALITY OF POINTS OF REDUCED CROSSSECTION OF SAID FUSIBLE ELEMENT BEING SITUATED OUTSIDE SAID SUB-CASING AND DIRECTLY IMMERSED IN SAID MASS OF QUARTZ SAND AND OPERATING AT A RELATIVELY LOW TEMPERATURE LEVEL WHEN SAID FUSIBLE ELEMENT IS CARRYING CURRENT. 